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“How many weeks pregnant am I?” has a slightly surprising answer: pregnancy is counted from the first day of your last menstrual period (LMP), which is about two weeks *before* you actually conceived. That’s the convention every doctor, scan and pregnancy book uses, so it’s the one this tool uses too.
Enter the first day of your last period and today’s date. You’ll get your gestational age in weeks and days, your current trimester, and your estimated due date.
How is it calculated?
Gestational age, in weeks and days
Gestational age is simply the time elapsed since your LMP, expressed as full weeks plus extra days — for example “14 weeks + 3 days”. The extra days matter because a single week spans several developmental changes.
| Value | Calculation |
|---|---|
| Days pregnant | Today − first day of last period |
| Weeks + days | Days ÷ 7 (whole weeks, remainder days) |
| Estimated due date | LMP + 280 days |
| Days remaining | 280 − days pregnant |
Why it’s about 2 weeks “ahead” of conception
At the start of a cycle you aren’t pregnant yet — ovulation and conception happen around two weeks in. But dating from the LMP is far more reliable than guessing the conception date, so by convention week 1 begins before conception. This is why a “40-week” pregnancy is really about 38 weeks of fetal development.
Trimesters
| Trimester | Weeks | Notes |
|---|---|---|
| First | 0 – 13 | Organs form; early scans and screening |
| Second | 14 – 27 | Anomaly scan; often the most comfortable stretch |
| Third | 28 – birth | Growth and preparation for delivery |
Accuracy
LMP dating assumes a regular cycle and a known period date. A first-trimester ultrasound measures the baby directly and is more precise; if it disagrees with the LMP date, clinicians re-date the pregnancy from the scan.
Worked example
If your last period began on 1 January 2026 and today is 9 April 2026, that’s 98 days — exactly 14 weeks + 0 days, placing you at the start of the second trimester. Your estimated due date is 1 January + 280 days = 8 October 2026, with about 182 days remaining. Because dating runs from the LMP, conception was around mid-January, so fetal development is roughly two weeks behind the “14 weeks pregnant” figure.
FAQ
How many weeks pregnant am I?+
Count the days from the first day of your last period to today, then divide by 7 for whole weeks plus leftover days — e.g. 98 days is 14 weeks + 0 days. This tool does it for you and also shows your trimester and due date.
Why is pregnancy dated from my last period and not conception?+
Because the LMP is a date you can identify reliably, while the exact day of conception usually isn’t known. Conception happens about two weeks after the LMP, so gestational age runs roughly two weeks ahead of the actual age of the fetus.
What trimester am I in?+
The first trimester is weeks 0–13, the second is 14–27, and the third is 28 until birth — all counted from your last period. The calculator shows your current trimester from the dates you enter.
How is the due date worked out?+
By adding 280 days (40 weeks) to the first day of your last period. It’s an estimate: a first-trimester ultrasound is more accurate and is used to re-date the pregnancy if it differs from the LMP calculation.
Is this a substitute for medical care?+
No. It’s an informational estimate based on the dates you enter, not medical advice. Your dates and due date should be confirmed by a healthcare provider, and prenatal care should follow their guidance.